There are ten Trek movies. When polls are made...the 2 Trek films that predominately come out as the best are "Wrath of Khan" and "First Contact".

Khan was the second film....Contact was the eighth.

Since it takes six more movies before one is considered better than Khan....it's rep is pretty good.

Khan is important to the franchise, because it changed the direction (uniforms, sets, secondary characters, etc...) that the first one had....and they continued until the end of the run of the "original" character's series (it's stuff was carried into the seventh movie).

So...Star Trek the Motion Picture started the movie franchise (and almost killed it)....and Khan managed to resurrect it, make it better, and continue for decades.